GB719093A - Production of artificial films - Google Patents

Abstract

<PICT:0719093/IV (a)/1> <PICT:0719093/IV (a)/2> A film of a synthetic linear polymer is obtained by extruding the polymer in molten state and passing the molten film downwardly into a pool of cooling liquid which is maintained in the nip formed between two contra-rotating rolls, the two rolls being rotated at equal peripheral speed and each roll, at the nip, being rotated in the direction of travel of the film which is passed through the nip. The nip rolls may be immersed to a depth less than their diameter in a bath of cooling liquid and a film of liquid may be allowed to travel round the rolls and into the nip to form and maintain the pool of cooling liquid. A wetting agent may be added to the liquid to ensure that sufficient is carried into the nip. Alternatively the pool may be formed by spraying the liquid on to the rolls or on to the molten film before the film reaches the nip. The cooling liquid maybe water, glycol or glycerol. Any tendency for the cooling liquid to be so heated by the molten film as to boil locally may be removed by providing cooling devices close to the surface of the film, e.g., refrigerator tubes, containing a chilled circulating liquid such as methanol, may be placed on each side of the film close to the surface thereof and extending across the width of the film. Alternatively, the nip rollers may be chilled by internal circulation of a chilled liquid. Cooling may also be accomplished by removing liquid, which has become heated, at each side of the film from the portions of the pool directly between the film and the rolls. This liquid may be removed at a plurality of points spaced across the width of the film, or it may be removed from a continuous area extending across the width of the film in a position as close as possible to the film. Cooling liquid may be allowed to flow from the ends of the nip at a rate controlled, e.g. by means of adjustable end plates, so as to give a pool of substantially constant depth in the nip. The polymer of which the film is formed may be a linear condensation polyester, such as polyethylene terephthalate, or a linear condensation polyamide. As s own in Figs. 1 and 2, the freshly extruded molten film 1 passes to the nip of steel rolls 2, which are immersed to a depth of about two thirds of their diameter in water contained in bath 3. Cold water supplied through inlet 4 flows through the bath and out by the adjustable overflow 5. Water is carried on the surface of the rolls to collect as a pool 6. The level of the liquid in the pool is maintained by end plates 7. Suction tubes 8 provided with a series of downwardly facing holes enable water to be drawn away from the pool in the region of the film surface. The process is carried out so that the rate of supply of cooling liquid to the pool is sufficient to ensure adequate cooling and solidification of the film but not so high as to produce turbulence at the surface of the pool. Films free from corrugations and surface blemishes may thus be obtained.